Download Economically Important Diseases of Spinach

James C. Correll and Teddy E. Morelock
'University of Arkansas, Fayettevi lle
Mark C. Black
Texas A&M University, Uvalde
Steven T. Kolke
University of California, Salinas
Lynn P. Brandenberger
Texas A&M University, Weslaco
Frank J. Dalnello
Texas A8M University, College Station
Economically Important Diseases of Spinach
Spinach (Spinacia oleracee L.)is an
economically important leafy vegetable
crop in many countries, and approximately 14,000 ha (35.000 acres) are
grown annually throughout the United
States (25) for fresh and processed
(frozen and canned) markets. The annual
crop value in the United States is
approximately $70 million (25). Major
spinach production states include Cab
ifarnia, Texas, Arkansas, Oklahoma,
Maryland, Virginia, New Jersey, and
Colorado (32).
Spinach apparently is native to central
Asia and has been cultivated for more
than 1,300 years (46). Spinach is a
member of the Chenopodiaceae and is
related to Swiss chard, sugar 'beet, and
table beet. The plant produces a compact
rosette of leaves and bolts (i.e., produces
a seed stalk) in response to temperature
and photoperiod. Spinach is usually
dioecious, producing either male or
female flowers. The majority of the
spinach seed used in the United States
is produced in the Pacific Northwest,
where long days correspond with moderate temperatures.
Spinach typically is grown as a directseeded crop, with planting and management strategies dictated by the market
destination of the commodity. Cvltivan
can vary from "flat-leaf' to "highIy
savoy" or wrinkled types. Spinach is
grown year-around in California but is
fall, winter, or spring crop in other
Dr. Correllk address is: Department of Plant Pathology, 217 Plan1 Scienccs Bu~ld~ng,
University of
Arkansas. Fayerrevillt, kR 72701.
.
a 1WTheAmerican Phytopathological Society
locations in the United States. Most
fresh-market spinach is hand-harvested
as a loose-leaf pack or bundled in containers. Spinach for processing is mechanically harvested and in some locations is repeat-harvested after regrowth.
As with most agricultural commodities, diseases impose significant production constraints affecting both yield and
overall quality of spinach. An integrated
disease management approach, including
the use of disease-resistant cultivars, crop
rotation, careful irrigation, fertility management. and fungicides, is often necessary to produce a high-quality product.
Although many diseases have been
reported on spinach (14,36,41), this discussion is limited to fungal and viral
diseases that are generally recognized as
economically important. Bacterial diseases of spinach in the field are noticeably
absent. Aster yellows, caused by a mycoplasma, is occasionally diagnosed (36),
and bacterial soft rot has been observed
as a postharvest disease of leaves and
on overly mature plants in the greenhouse.
Downy Mlldew
Downy mildew (or blue mold), caused
by Peranaspora furinosa (Fr.:Er.) Fr. f.
sp. spinaciae Byford (= P. cfltlsa (Grev.)
Ces.) (6) is probably the most widespread
and potentially destructive disease of
spinach worldwide. Initial symptoms are
slightly yellow, irregular, chlorotic
lesions on leaves (Fig. I A ) . Lesions
frequently expand and coalesce and may
become necrotic. Heavily infected teaves
can appear curled and distorted. Under
wet conditions a n d / o r high relative
humidity, blue-gray sporangia a n d
sporangiospores are produced and can
be seen in mass on the underside of the
leaf and occasianally on the upper leaf
surface (Fig. I B). Under environmental
conditions favorable for the fungus,
sporulation can often be observed
in the absence of lesion development.
P. J spinaciae is a hcterothallic obligate fungal pathogen belonging to the
Peronosporales (18). Although it is
rnorphologicalIy similar to t h e downy
miIdew pathogen that attacks beets, the
two apparentIy are physiologicalEy
distinct. with P. j: spinociae infecting
only the genus Spinaria and several
species of Chmopodium (9). Four physiologically distinct races of P.f. spine ria^
have been reported (Table 1) (6).
Under coal, wet conditions. sporangia
can germinate directly or release zoospores in 2-6 hours on leaf surfaces (3 I).
Germination of sporangia and gem tube
elongation can occur between 2 and 25
C, w ~ t han optimum temperature of9-12
C. Lesion development is favored by
temperatures of 15-25 C. and resporulation can occur 6- 12 days after infection.
Sporangia are readily dispersed by wind
and rain splashing but can rapidly lose
viability when desiccated or exposed to
suntight (3 1 -45). Under conditions of
prolonged leaf wetness and cool temperatures, epidemics can progress very
rapidly and an entire crop can be lost
in a short period of time. As lesions
mature, the fungus can produce oospores
that serve as a resistant survival structure. Oospores and mycelium of the
downy mildew pathogen have been recovered from spinach seed, and contaminated seed have been shown to give rise
to infected seedlings (19). Oospores can
survive in sail and may represent an
important source of primary inoculum
(45). Windborne sporangia from surPlant DiseaseIJuly f 994
653